The present invention relates to a bi-directional digital transmission system and a bi-directional digital transmission method, and in particular to a bi-directional digital transmission system and a bi-directional digital transmission method using an orthogonal frequency division multiplex modulation method.
For terrestrial transmission of moving picture signals, FPU (Field Pick Up) communication devices (hereafter referred to as FPU) are wide spread. In order to determine whether moving pictures of a main signal transmitted from a mobile station having a camera and so on to a base station such as a broadcasting center are transmitted correctly, a return signal for returning moving pictures received by a base station to a mobile station is in great demand in applications of mobile radio or semi-fixed radio. Actually, however, the receiving state of the moving pictures are conveyed from the base station to the mobile station by using communication means such as telephone.
A FPU capable of conducting the bi-directional transmission may be implemented by using two transmission bands. At the same time, however, there are also some problems in the implementation using such two transmission bonds.
FIG. 12 shows a configuration example of a FPU capable of conducting the bi-directional transmission devised by the present inventors.
Hereafter, the configuration and problems of the FPU capable of conducting bi-directional transmission will be described by referring to FIG. 12.
In FIG. 12, a sending control unit 21, a sending high frequency unit 25, an up-sending antenna 30, an up-receiving antenna 32, a receiving high frequency unit 27, and a receiving control unit 23 form an up-transmission system. A sending control unit 24, a sending high frequency unit 28, a sending antenna 33, a receiving antenna 31, a receiving high frequency unit 26, a receiving control unit 22 form a down-transmission system.
In operation, the sending control units 21 and 24 have the same configuration. A sending data rate is determined by a clock oscillator CK OSC. Data input to an ID terminal is subjected to digital modulation, then modulated to an IF signal having a frequency determined by a local oscillator LO OSC, and outputted. The input sending high frequency unit 25 or 28 converts the IF signal supplied from the sending control unit 21 or 24 to a specified RF frequency. The converted output is sent by the antenna 30 or 33. The receiving high frequency unit 26 or 27 receives only the RF signal of the specified frequency from the antenna 32 or 31, and converts it to an IF signal. The receiving control unit 23 or 22 demodulates the inputted received IF signal by using the frequency of the local oscillator LO VCO, and then outputs a digital signal having a rate determined by the output of the clock oscillator CK VCO. In this case, the radio wave used for the down-direction uses a frequency different from the frequency of the up-direction.
In the above-described bi-directional transmission system, there is a problem that two frequency bands are required and limited radio resources cannot be used effectively. In addition, the sending antenna and the receiving antenna of the up-transmission system and the sending antenna and the receiving antenna of the down-transmission system are required. In other words, four antennas in total are required.
Therefore, an object of the present invention is to remove these drawbacks and provide a transmission method, and transmission apparatus, using an orthogonal frequency division multiplex modulation scheme in which bi-directional transmission is conducted by using one frequency band.
Another object of the present invention is to provide a transmission method, and transmission apparatus, using an orthogonal frequency division multiplex modulation in which bi-directional transmission is conducted by using two sending/receiving antennas.
In order to achieve the above described objects, the bi-directional transmission of the present invention is based on the operation principle of conducting switchover between the up-direction transmission and down-direction transmission on the time axis, that is, carrying out up-direction transmission and down-direction transmission alternately with respect to time. In each transmission station on mobile station side and base station side, continuous data is converted to a digital signal which is modulated by an orthogonal frequency division multiplex modulation and is made up of a repetition of a signal sending period (signal carrying section) and a signal non-sending period (signal non-carrying section) that appear alternately so that the up-direction intermittent digital signal and the down-direction intermittent signal are transmitted alternately in terms of time.
In accordance with one aspect of the present invention, a bi-directional digital transmission method for sending/receiving digital transmission signals having a frame configuration modulated by an orthogonal frequency division multiplex modulation scheme between a first and a second transmission station, comprising the steps of:
controlling the first and second transmission stations so as to alternately produce a first sending period to be used by the first transmission station for transmission and a second sending period to be used by the second transmission station for transmission, the first sending period and subsequent second sending period constituting one frame;
converting first continuous data to a up-direction intermittent transmission signal of a frame configuration having a sending section and a non-sending section appearing alternately and further orthogonal-frequency-division-multiplex-modulating the up-direction intermittent transmission signal so as to send digital information in the sending section from the first transmission station during the first sending period;
converting second continuous data to a down-direction intermittent transmission signal of a frame configuration having a sending section and a non-sending section appearing alternately and further orthogonal-frequency-division-multiplex-modulating the down-direction intermittent transmission signal so as to send digital information in the sending section from the second transmission station during the second sending period;
receiving the digital information from the first transmission station by the second transmission station and restoring the original first continuous data; and
receiving the digital information from the second transmission station by the first transmission station and restoring the original second continuous data.
In accordance with another aspect of the present invention, a bi-directional digital transmission system for transmitting digital transmission signals having a frame configuration modulated by an orthogonal frequency division multiplex modulation scheme includes:
a first transmission station including a first sending unit, a first receiving unit, a first switchover unit connected to an output of the first sending unit and an input of the first receiving unit, selection of either the output of the first sending unit or the input of the first receiving unit being controlled by the first sending unit, and a first sending/receiving path connected to the selected output of the first switchover unit; and
a second transmission station including a second sending unit, a second receiving unit, a second switchover unit connected to an output of the second sending unit and an input of the second receiving unit, selection of either the output of the second sending unit or the input of the second receiving unit being controlled by the second receiving unit, and a second sending/receiving path connected to the selected output of the second switchover unit,
wherein in the first transmission station the first sending unit converts first continuous data to a up-direction intermittent transmission signal and sends it from the first sending/receiving path via the first switchover unit during a first sending period, and in the second transmission station the second receiving unit restores the original first continuous data from the up-direction intermittent transmission signal received by the second sending/receiving path and supplied via the second switchover unit, and
in the second transmission station the second sending unit converts second continuous data to a down-direction intermittent transmission signal and sends it from the second sending/receiving path via the second switchover unit during a second sending period, and in the first transmission station the first receiving unit restores the original second continuous data from the down-direction intermittent transmission signal received by the first sending/receiving path and supplied via the first switchover unit, the first and second sending periods appear alternately and repetitively.
In accordance with still another aspect of the present invention, a bi-directional digital transmission method for sending/receiving digital transmission signals having a frame configuration modulated by an orthogonal frequency division multiplex modulation scheme between a first transmission station and a second transmission station, the bi-directional digital transmission method includes the steps of:
controlling the first and second transmission stations so as to alternately produce a first sending period to be used by the first transmission station for transmission and a second sending period to be used by the second transmission station for transmission, the first sending period and subsequent second sending period constituting one frame;
converting first continuous data to a up-direction intermittent transmission signal of a frame configuration having a sending section and a non-sending section appearing alternately and further orthogonal-frequency-division-multiplex modulating the up-direction intermittent transmission signal in a first sending unit in the first transmission station so as to send digital information in the sending section from a first antenna via a first switchover unit during the first sending period;
receiving the up-direction intermittent transmission signal by a second antenna of the second transmission station;
supplying the receiving up-direction intermittent transmission signal to a second receiving unit the second transmission station via a second switchover in the second transmission station and restoring the original first continuous data;
converting second continuous data to a down-direction intermittent transmission signal of a frame configuration having a sending section and a non-sending section appearing alternately and further orthogonal-frequency-division-multiplex modulating the down-direction intermittent transmission signal by a second sending unit in the second transmission station so as to send digital information in the sending section from a second antenna via a second switchover unit during the second sending period;
receiving the down-direction intermittent transmission signal by the first antenna in the first transmission station; and
supplying the received down-direction intermittent transmission signal to a first receiving unit in the first transmission station via the first switchover unit and restoring the original second continuous data from the received down-direction intermittent transmission signal.
Preferably, the clock oscillation output of the up-transmission system is used as a reference clock of the down-transmission system as well in order that the data rate of the down-transmission system may be determined by the up-transmission system.